This invention related to detectors of optical radiation (i.e., lightwaves) and, more particularly, to semiconductor photodiodes.
The recent special issue of the Western Electric Engineer, Vol. XXIV, No. 1, Winter 1980, is a graphic illustration of the burgeoning interest in lightwave communication systems, especially fiber optic systems. The rapid growth of these systems has engendered commensurate activity in optical sources and detectors, primarily GaAs-AlGaAs laser diodes and LEDs in conjunction with Si APDs and p-i-n diodes for present applications at relatively short wavelengths (e.g., 0.80-0.90 .mu.m), and InP-InGaAsP laser diodes and InGaAs photodiodes for future systems at longer wavelengths (e.g., 1.1-1.6 .mu.m).
In these systems, the receiver sensitivity is limited by the dark current of the p-i-n photodiode, especially at low bit rates. For example, at bit rates less than about 12 MHz, the room temperature photodiode dark current should be less than about 5 nA at the operating voltage so that it will remain sufficiently low to avoid degrading the sensitivity of a GaAs-FET receiver at elevated operating temperatures.
Because of their relatively low dark current densities and room temperature bandgap of 0.75 eV, In.sub.0.53 Ga.sub.0.47 As p-i-n photodiodes are particularly well suited as optical detectors in the 1.0-1.6 .mu.m range. Recent reports in the technical literature provide ample evidence of the intensity of activity directed toward the fabrication of practical In.sub.0.53 Ga.sub.0.47 As p-i-n photodiodes. For example, FIG. 1 shows a mesa-type In.sub.0.53 Ga.sub.0.47 As p-i-n photodiode in which the p-n junction is formed at the interface between p-type and n-type InGaAs epitaxial layers, and the mesa is formed by standard etching down to the InP substrate. FIG. 1 is representative of devices reported by T. P. Pearsall, IEEE Journal of Quantum Electronics, Vol. QE-16, p. 709, (1980); R. F. Leheny et al, Electronics Letters, Vol. 15, p. 713, (1979); T. P. Lee et al, Electronics Letters, Vol. 16, p. 155, (1980); and Y. Matsushima, Applied Physics Letters, Vol. 35, p. 466, (1979). Although a few of these photodiodes from a typical wafer may have relatively low dark currents (.ltoreq.5 nA), they more typically have dark currents of .about.10 nA. In addition, they have not been adequately passivated, making them extremely sensitive to humidity, oxygen and other atmospheric conditions, and their reliability is relatively poor.
On the other hand, planar p-i-n photodetectors, as shown in FIG. 2, are more amenable to passivation and reproducible fabrication. The planar structure has only one passivating silicon nitride layer which also serves as a mask for Zn-diffusion for forming the p-n junction. Such devices, even though easier to fabricate than mesa-type structures, have typical dark current values of tens of nanoamperes and questionable reliability. FIG. 2 is representative of photodiodes reported by N. Susa et al, IEEE Electron Device Letters, EDL-1, p. 55, (1980); N. Susa et al, Electronics Letters, Vol. 15, p. 238, (1979); and S. R. Forrest et al, IEEE Electron Device Letters, Vol. EDL-2, p. 283, (1981).